3GPP LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station transmits a synchronization signal (Synchronization Channel: SCH) and a broadcast signal (Broadcast Channel: BCH) using predetermined communication resources. A terminal secures synchronization with the base station by catching the SCH first. The terminal then reads BCH information and thereby acquires parameters (e.g. frequency bandwidth) peculiar to the base station (see Non-Patent Literatures 1, 2, 3).
Furthermore, after completing the acquisition of the parameters peculiar to the base station, the terminal requests the base station for a connection and thereby establishes communication with the base station. The base station transmits control information to the terminal with which communication is established via a PDCCH (Physical Downlink Control CHannel) as required.
The terminal then makes a “blind detection” on a plurality of pieces of control information included in the received PDCCH signal. That is, the control information includes a CRC (Cyclic Redundancy Check) portion and this CRC part is masked with a terminal ID of the destination terminal in the base station. Therefore, the terminal cannot decide whether the control information is directed to the terminal or not until the terminal demasks the CRC part of the received control information using the terminal ID of the terminal. In this blind detection, when the demasking result shows that the CRC calculation results in OK, the control information is decided to be directed to the terminal.
Furthermore, 3GPP LTE applies ARQ (Automatic Repeat Request) to downlink data from the base station to the terminal. That is, the terminal feeds back a response signal indicating an error detection result of the downlink data to the base station. The terminal performs a CRC on the downlink data and feeds back ACK (Acknowledgment) when CRC=OK (no error) or NACK (Negative Acknowledgment) when CRC=NG (error present) to the base station as a response signal. An uplink control channel such as a PUCCH (Physical Uplink Control Channel) is used to feed back this response signal (that is, ACK/NACK signal).
Here, the above-described control information transmitted from the base station includes resource assignment information including resource information or the like assigned to the terminal by the base station. The above-described PDCCH is used to transmit the control information. The PDCCH is constructed of one or a plurality of L1/L2 CCHs (L1/L2 Control Channel). Each L1/L2 CCH is constructed of one or a plurality of CCEs (Control Channel Elements). That is, the CCE is a base unit when control information is mapped to the PDCCH. Furthermore, when one L1/L2 CCH is constructed of a plurality of CCEs, a plurality of continuous CCEs are assigned to the L1/L2 CCH. The base station assigns an L1/L2 CCH to a resource assignment target terminal according to the number of CCEs necessary to report control information to the resource assignment target terminal. The base station then transmits the control information mapped to physical resources corresponding to the CCEs of the L1/L2 CCH.
Furthermore, here, the CCEs are associated with component resources of a PUCCH in a one-to-one correspondence. Therefore, the terminal that receives the L1/L2 CCH identifies component resources of the PUCCH corresponding to CCEs making up the L1/L2 CCH and transmits a response signal to the base station using the resources. Downlink communication resources are thereby efficiently used.
A plurality of response signals transmitted from a plurality of terminals are spread on the time axis by a ZAC (Zero Auto-correlation) sequence having Zero Auto-correlation characteristics, Walsh sequence and DFT (Discrete Fourier Transform) sequence and code-multiplexed within a PUCCH as shown in FIG. 1. In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4 and (F0, F1, F2) represents a DFT sequence having a sequence length of 3. As shown in FIG. 1, in the terminal, a response signal of ACK or NACK is primary-spread within 1 SC-FDMA symbol by a ZAC sequence (sequence length 12) on the frequency axis first. Next, the primary-spread response signals are associated with W0 to W3, F0 to F3 respectively and subjected to IFFT (Inverse Fast Fourier Transform). The response signal spread by the ZAC sequence having a sequence length of 12 on the frequency axis is transformed into the ZAC sequence having a sequence length of 12 on the time axis through IFFT. The signal after the IFFT is further secondary-spread using a Walsh sequence (sequence length 4) and DFT sequence (sequence length 3).
Furthermore, standardization of 3GPP LTE-advanced has been started which realizes still faster communication than 3GPP LTE. A 3GPP LTE-advanced system (hereinafter also referred to as “LTE-A system”) follows the 3GPP LTE system (hereinafter also referred to as “LTE system”). In order to realize a downlink transmission rate of maximum 1 Gbps or above, 3GPP LTE-advanced is expected to introduce base stations and terminals capable of communicating at a wideband frequency of 40 MHz or above.
In an LTE-A system, to realize communication at an ultra-high speed several times as fast as the transmission rate in an LTE system and backward compatibility with the LTE system simultaneously, a band assigned to the LTE-A system is divided into “unit bands” of 20 MHz or less which is a support bandwidth of the LTE system. That is, the “unit band” is a band having a width of maximum 20 MHz and defined as a base unit of a communication band. Furthermore, the “unit band” (hereinafter referred to as “downlink unit band”) in a downlink may be defined as a band divided by downlink frequency band information in a BCH broadcast from the base station or by a spreading width when the downlink control channel (PDCCH) is arranged by being spread in the frequency domain. Furthermore, the “unit band” (hereinafter referred to as “uplink unit band”) in an uplink may be defined as a band divided by uplink frequency band information in a BCH broadcast from the base station or as a base unit of a communication band of 20 MHz or less including a PUSCH (Physical Uplink Shared CHannel) field near the center and PUCCHs for LTE at both ends. Furthermore, in 3GPP LTE-Advanced, the “unit band” may also be expressed as “component carrier(s)” in English.
The LTE-A system supports communication using a band that bundles several unit bands, so-called “carrier aggregation.” Since throughput requirements for uplinks and throughput requirements for downlinks are generally different, in the LTE-A system, studies are being carried out on carrier aggregation in which the number of unit bands set for an arbitrary LTE-A system compatible terminal (hereinafter referred to as “LTE-A terminal”) differs between the uplink and downlink, so-called “asymmetric carrier aggregation.” Furthermore, cases are also supported where the numbers of unit bands are asymmetric between the uplink and downlink, and the frequency bandwidth differs from one unit band to another.
FIG. 2 is a diagram illustrating asymmetric carrier aggregation and a control sequence applicable to individual terminals. FIG. 2 shows an example where bandwidths and the number of unit bands are symmetric between the uplink and downlink of the base station.
In FIG. 2, a setting (configuration) is made for terminal 1 so as to perform carrier aggregation using two downlink unit bands and one uplink unit band on the left side, whereas although a setting is made for terminal 2 so as to use two downlink unit bands identical to those of terminal 1, a setting is made in uplink communication so as to use the uplink unit band on the right side.
Focusing attention on terminal 1, signals are transmitted/received between an LTE-A base station and an LTE-A terminal making up an LTE-A system according to the sequence diagram shown in FIG. 2A. As shown in FIG. 2A, (1) terminal 1 establishes synchronization with the downlink unit band on the left side at a start of communication with the base station and reads information of the uplink unit band which forms a pair with the downlink unit band on the left side from a broadcast signal called “SIB 2 (System Information Block Type 2).” (2) Using this uplink unit band, terminal 1 starts communication with the base station by transmitting, for example, a connection request to the base station. (3) Upon deciding that a plurality of downlink unit bands need to be assigned to the terminal, the base station commands the terminal to add downlink unit bands. In this case, however, the number of uplink unit bands does not increase and terminal 1 which is an individual terminal starts asymmetric carrier aggregation.